1. Field of the Invention
The present invention relates to novel fluorochemical surfactants and in particular, it relates to the use of novel fluorochemical surfactants in photothermographic and thermographic elements. The use of fluorochemical surfactants in coating compositions reduces disuniformities such as mottle in photothermographic and thermographic coatings.
2. Background of the Art
Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic elements) processed with heat, and without liquid development, have been known in the art for many years. These materials are also known as "dry silver" compositions or emulsions and generally comprise a support having coated thereon: (a) a photosensitive compound that generates silver atoms when irradiated; (b) a non-photosensitive, reducible silver source; (c) a reducing agent (i.e., a developer) for silver ion, for example the silver ion in the non-photosensitive, reducible silver source; and (d) a binder.
The photosensitive compound is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver atoms (also known as silver specks, clusters, or nuclei) are generated by irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that silver atoms (Ag.degree.) are a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions. The silver halide may be made "in situ" for example, by adding a halogen-containing source to the reducible silver source to achieve partial metathesis (see, for example, U.S. Pat. No. 3,457,075); or by coprecipitation of silver halide and the reducible silver source material (see, for example, U.S. Pat. No. 3,839,049). The silver halide may also be made "ex situ" and added to the organic silver salt. The addition of silver halide grains to photothermographic materials is described in Research Disclosure, June 1978, Item No. 17029. It is also reported in the art that when silver halide is made ex situ, one has the possibility of controlling the composition and size of the grains much more precisely so that one can impart more specific properties to the photothermographic element and can do so much more consistently than with the in situ technique.
The non-photosensitive, reducible silver source is a material that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further processed to produce a visible image. This is accomplished by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, (i.e., the latent image). This produces a black and white image. In photographic elements, the silver halide is reduced to form the black-and-white image. In photothermographic elements, the light-insensitive silver source is reduced to form the visible black-and-white image while much of the silver halide remains as silver halide and is not reduced.
In photothermographic and thermographic elements the reducing agent for the organic silver salt, often referred to as a "developer", may be any material, preferably any organic material, that can reduce silver ion to metallic silver. At elevated temperatures, in the presence of the latent image, the non-photosensitive reducible silver source (e.g., silver behenate) is reduced by the reducing agent for silver ion. This produces a negative black-and-white image of elemental silver.
While conventional photographic developers such as methyl gallate, hydroquinone, substituted-hydroquinones, catechol, pyrogallol, ascorbic acid, and ascorbic acid derivatives are useful, they tend to result in very reactive photothermographic formulations and fog during preparation and coating of photothermographic elements. As a result, hindered phenol reducing agents have traditionally been preferred.
As the visible image in black-and-white photothermographic and thermographic elements is usually produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable to reduce the cost of raw materials used in the emulsion and/or to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic elements as described in U.S. Pat. Nos. 3,846,136; 3,994,732; and 4,021,249.
Another method of increasing the maximum image density in photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming or dye-releasing materials in the emulsion. Upon imaging, the dye-forming or dye-releasing material is oxidized, and a dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye-enhanced black-and-white silver image can be produced.
Thermographic imaging constructions (i.e., heat-developable materials) processed with heat, and without liquid development, are widely known in the imaging arts and rely on the use of heat to help produce an image. These elements generally comprise a support or substrate (such as paper, plastics, metals, glass, and the like) having coated thereon: (a) a thermally-sensitive, reducible silver source; (b) a reducing agent for the thermally-sensitive, reducible silver source (i.e., a developer); and (c) a binder.
In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, silver behenate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechol, pyrogallol, ascorbic acid, ascorbic acid derivatives, and the like, whereby an image comprised of elemental silver is formed.
Many times, the thermographic construction is brought into contact with the thermal head of a thermographic recording apparatus, such as a thermal printer, thermal facsimile, and the like. In such instances, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of about 60.degree.-225.degree. C., resulting in the formation of an image.
The imaging arts have long recognized that the fields of photothermography and thermography are clearly distinct from that of photography. Photothermographic and thermographic elements differ significantly from conventional silver halide photographic elements which require wet-processing
In photothermographic and thermographic imaging elements a visible image is created by heat as a result of the reaction of a developer incorporated within the element. Heat is essential for development and temperatures of over 100.degree. C. are routinely required. In contrast, conventional wet-processed photographic imaging elements require processing in aqueous processing baths to provide a visible image (e.g., developing and fixing baths) and development is usually performed at a more moderate temperature (e.g., 30.degree.-50.degree. C.).
In photothermographic elements only a small amount of silver halide is used to capture light and a different form of silver (e.g., silver behenate) is used to generate the image with heat. Thus, the silver halide serves as a catalyst for the development of the non-photosensitive, reducible silver source. In contrast, conventional wet-processed black-and-white photographic elements use only one form of silver (e.g., silver halide) which, upon development, is itself converted to the silver image. Additionally, photothermographic elements require an amount of silver halide per unit area that is as little as one-hundredth of that used in conventional wet-processed silver halide.
Photothermographic systems employ a light-insensitive silver salt, such as silver behenate, which participates with the developer in developing the latent image. In contrast, photographic systems do not employ a light-insensitive silver salt directly in the image-forming process. As a result, the image in photothermographic elements is produced primarily by reduction of the light-insensitive silver source (silver behenate) while the image in photographic black-and-white elements is produced primarily by the silver halide.
In photothermographic and thermographic elements all of the "chemistry" of the system is incorporated within the element itself. For example, photothermographic and thermographic elements incorporate a developer (i.e., a reducing agent for the non-photosensitive reducible source of silver) within the element while conventional photographic elements do not. The incorporation of the developer into photothermographic elements can lead to increased formation of "fog" upon coating of photothermographic emulsions as compared to photographic emulsions. Even in so-called instant photography, developer chemistry is physically separated from the silver halide until development is desired. Much effort has gone into the preparation and manufacture of photothermographic and thermographic elements to minimize formation of fog upon coating, storage, and post-processing aging.
Similarly, in photothermographic elements, the unexposed silver halide inherently remains after development and the element must be stabilized against further development. In contrast, the silver halide is removed from photographic elements after development to prevent further imaging (i.e., the fixing step).
In photothermographic and thermographic elements the binder is capable of wide variation and a number of binders are useful in preparing these elements. In contrast, photographic elements are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic and thermographic elements require thermal processing, they pose different considerations and present distinctly different problems in manufacture and use. In addition, the effects of additives (e.g., stabilizers, antifoggants, speed enhancers, sensitizers, supersensitizers, etc.) which are intended to have a direct effect upon the imaging process can vary depending upon whether they have been incorporated in a photothermographic or thermographic element or incorporated in a photographic element.
Distinctions between photothermographic and photographic elements are described in Imaging Processes and Materials (Neblette's Eighth Edition); J. Sturge et al. Ed; Van Nostrand Reinhold: New York, 1989; Chapter 9 and in Unconventional Imaging Processes; E. Brinckman et al, Ed; The Focal Press: London and New York: 1978; pp. 74-75.
Photothermographic and thermographic constructions are usually prepared by coating from solution and removing most of the coating solvent by drying. One common problem that exists with coating photothermographic systems is the formation of coating defects. Many of the defects and problems that occur in the final product can be attributable to phenomena that occur in the coating and drying procedures. Among the problems that are known to occur during drying of polymeric film layers after coating are unevenness in the distribution of solid materials within the layer. Examples of specific types of coating defects encountered are "orange peel", "mottling", and "fisheyes". Orange peel is a fairly regular grainy surface that occurs on a dried coated film, usually because of the action of the solvent on the materials in the coating composition. Mottling often occurs because of an unevenness in the removal of the solvent from the coating composition. "Fisheyes" are another type of coating problem, usually resulting from a separation of components during drying. There are pockets of different ingredients within the drying solution, and these pockets dry out into uneven coating anomalies.
Surfactants have often been used to correct these types of problems, along with changes in the solvents of the coating compositions. In some cases, surfactants do not correct the problem, and in other cases the surfactants create other problems even when they cure the first problem. It is sometimes necessary to investigate a large number of commercially available surfactants before finding one that is appropriate for a particular type of system, even if that commercial product is touted for use in correcting a particular type of defect.
For a surfactant to be useful in an imaging element is must have several properties. It must be soluble in the coating solution or emulsion. If it were not, then other defects such as "fisheyes" and streaks may occur in the dried coating. The surfactant must not stabilize foams or air bubbles with the coating solution or emulsion as these cause streaks in the dried coating. These defects are readily visible and are unacceptable in a final element. Additionally, the surfactant cannot significantly alter the sensitometric properties of the imaging element such as speed, contrast, minimum density, and maximum density.
Fluorochemical surfactants are useful in coating applications to reduce mottle. When a coating solution is dried at high speeds in an industrial oven, the resulting film often contains a mottle pattern. This mottle pattern is often the result of surface tension gradients created by non-uniform drying conditions. When an appropriate fluorochemical surfactant is added to the coating solution, the surfactant holds the surface tension at a lower but constant value. This results in a uniform film, free from mottle. Fluorochemical surfactants are used because organic solvents such as 2-butanone (also known as methyl ethyl ketone or MEK) already have such low surface energies (24.9 dyne/cm) that hydrocarbon surfactants are ineffective.
U.S. Pat. Nos. 4,764,450 and 4,853,3 14 describe the use of particular changes in solvent systems to improve surface defects in positive-acting photoresist imaging systems.
U.S. Pat. No. 4,557,837 describes fluorochemicals useful in the preparation of foamable compositions such as those used in the cleanup of gas wells. Polymers described include copolymers of fluorochemical monomers and hydroxyethylacrylate, and copolymers of fluorochemical monomers, acrylic acid, and short chain acrylates.
JP 01-223,168 describes fluorinated terpolymers that are useful additives to varnish formulations. They improve the stain resistance of the varnish.
JP 57-040579 describes fluorinated terpolymers which are useful as release coatings for adhesive tapes.
U.S. Pat. No. 3,885,965 describes the use of poly(dimethylsiloxane) to resist "orange peel" effects in photothermographic elements.
U.S. Pat. No. 3,950,298 describes thermoplastic fluorinated terpolymers that are useful as non-foaming additives to coating solutions for polymeric materials such as carpets and fibers. The coating compositions provide oleophobicity to the surfaces that are coated.
U.S. Pat. No. 4,365,423 describes a process where a foraminous shield (such as a screen or perforated plate) is used to protect the coated web from the impingement air used for dying. Both solvent-rich and solvent-poor air can flow through the shield. Air velocity and turbulence are reduced by the porous shield. Although this method is claimed to reduce the degree of mottle, the amount and presence of mottle was still influenced by increased flow rate of the impingement air.
U.S. Pat. No. 4,999,927 describes an oven system for which the air flow boundary layer along the web remains laminar. This is accomplished by accelerating the air through the drying chamber.
U.S. Pat. No. 4,894,927 describes a technique for reducing mottle by combining an inert gas system with a small drying chamber. Using this method, the air flow remains laminar over the web.
U.S. Pat. No. 3,573,916 describes the use of sulfo-substituted cyanine dyes to reduce mottle in color-bearing silver halide emulsions which have been coated on electron bombarded hydrophobic surfaces.
U.S. Pat. No. 5,270,378 describes the use of fluorochemical surfactants to reduce coating disuniformities such as mottle, fisheye, and foaming in positive-acting or negative-acting resist systems such as printing plates and other non-resist imageable polymerizable systems. These polymers are comprise a fluorochemical acrylate, a short-chain-alkyl acrylate, and a polar monomer. Use of these materials in photothermographic or thermographic elements is not discussed.
U.S. Pat. No. 5,380,644 describes the use of fluorinated terpolymers having at least three different groups within the polymer chain. The groups are derived from a) a fluorinated, ethylenically unsaturated monomer, b) a hydroxyl-containing ethylenically unsaturated monomer, and c)a polar, ethylenically unsaturated monomer. The fluorinated terpolymers formed by the polymerization of the above mentioned monomers provide a surfactant that is particularly useful in the coating of photothermographic and thermographic elements. The surfactants can reduce surface anomalies such as mottle when used with certain solvent systems.